The present invention relates to image processing methods. More specifically, the present invention relates to an image processing method for dither images.
Most of output apparatuses such as display apparatuses and printing apparatuses in present use display images in only two colors, namely, white and black. A density pattern method (luminance pattern method) and dither pattern method are known as prior arts to express halftones in a pseudo-way using the above described output apparatuses. Both density pattern method and dither pattern method are one type of area gradation methods which vary the number of dots to be recorded in a certain area (matrix).
As shown in FIG. 30-b, the density pattern method records an area corresponding to a pixel in a document with a plurality of dots using a threshold matrix. The dither method records an area corresponding to a pixel in a document with one dot as shown in FIG. 30-a. Dither outputs are obtained according to these two methods as shown in FIGS. 30-a and 30-b. These outputs express halftone images in a pseudo-way with and black dots.
If these binary, pseudo halftone images may be reconverted to original halftone images (corresponding to the input data shown in FIG. 30), the process enables various types of data processing, thus providing a wide range of varieties in image conversion process. In the case of the density pattern method, detection of pattern level arrangement can immediately reconvert a pseudo halftone image to an original halftone image. However the resolution performance is low, in comparison with the input data amount. On the contrary, the resolution performance of the dither method is higher than that of the density method. However it is difficult to reconvert the pseudo halftone images to original halftone images by means of the dither method. Therefore, various image conversions can not be accomplished only by the dither method.
The methods for enlarging or reducing a binary image known in the prior art are as follows:
(1) SPC method
This method selects a pixel on a document located closet to a pixel to be converted and determines the density value of the selected pixel as a value of the pixel to be converted.
(2) Logical sum method
This method outputs Ir, which is a pixel density to be converted, as a logical sum of the densities of Ia, Ib, Ic, and Id of four pixels on a document closest to a pixel to be converted. That is, the Ir is expressed as follows: EQU Ir=Ia U Ib U Ic U Id (1)
(3) 9 division method
This method decides a density Ir of a pixel to be converted using a predetermined logical operation formula, where in a square area on a document whose vertexes are original pixels A, B, C, and D on the document is devided into 9 regions, G1 through G9, and the logical formula equation is determined according to the region Gm (m=1 through 9) including the pixel to be converted. For example, assuming that m=8, Ir is determined by the following formula: EQU Ir=Id U Ia (2)
(4) Projective method
This method determines an average density fr of an original pixel which is projected on a pixel to be converted, and then, the fr value is processed with a threshold value to obtain a conversion pixel value Ir.
The process to enlarge or reduce a binary image by the SPC method is simple. However this method has a disadvantage that omissions are likely to occur owing to disconnections in strokes caused by the likelihood that the strokes are narrowed when the binary image is reduced. Although the strokes are made thicker by the logical sum method, this method also has a disadvantage that unnecessary connections of neighboring strokes occur when a binary image is reduced, thus the image is deformed. By making the strokes thicker when a binary image is reduced, the 9 division method ensures a clear image. However, the method is also disadvantageous in that the strokes are also made thicker when the binary image is enlarged, thus the image quality is degraded. Finally, the projection method reduces omissions and deformations in a converted image to a very low level, and the original image is favorably enlarged or reduced with an accurate similarity. However, this method has a drawback that it takes a long period to process the binary image because of the large amount of data to be arithmetically processed.
A reducing or enlarging process of a binary image such as a dither image by means of a prior art causes disadvantageous phenomena such as moire fringe generation, lowering of resolution performance, variation of gradation pattern, and the like. Further, characters are deformed when enlarging a binary image of characters and line drawings, and fine lines are blurred when the image is reduced, thus the quality of converted binary images is greatly degraded.